We propose to apply a unique technology to the study of diabetic retinal vascular disease. Digitized images from a fundus camera provide a reproducible means of accessing venous blood oxygenation (oximetry), vessel diameter and circulation time. Alterations in retinal vascular circulation time, reactivity, and oxygen content are thought to contribute to the development of diabetic retinopathy. We propose to calibrate the system using capillary tubes and subhuman primates and to study the effects of photocoagulation on the monkeys. We would then study changes in the retinal circulation in normal subjects and in patients with diabetic retinopathy. We propose to compare adult-onset and juvenile-onset diabetics, to examine the effects of changes in metabolic control and to evaluate the retinal vascular effects of pan-retinal photocoagultion and aspirin therapy in patients with diabetes.